The advantages of using low temperatures for crystallography are well understood. Several commercially available systems such as the Oxford Cryostream use liquid nitrogen as a refrigerant and are relatively easy to use in conjunction with traditional methods of single crystal crystallography. However, a system which uses liquid helium as a refrigerant is much better than a nitrogen system because of the ability to achieve even lower temperatures which ultimately improves the quality of the crystallography (the minimum stable temperature achievable by a system using liquid nitrogen is about 85K; in contrast, a liquid helium system is capable achieving temperatures of 30K or lower). Unfortunately, the use of liquid helium as a refrigerant creates its own unique set of problems.
One problem with the use of helium as a refrigerant is the level of expertise required to operate a traditional system, as well as the cost to build it. For example, a typical system as used in the past involved mounting a closed cycle helium refrigerator on a large diffractometer, using, for example, a large Huber circle. The systems are complex in nature, thus requiring a high level of training for operators. Furthermore, a window made of beryllium is required which obscures the crystal and introduces systematic error into the data.
Another problem associated with the use of liquid helium as a refrigerant is the likelihood of frost or solid air forming on and around the inner nozzle of the cryostat or on the crystal itself or its support structure which inhibits accurate examining of the crystal. Frost/solid air formation is caused by the freezing of the gaseous components of air at the very low working temperatures used.
Based on the above, it is clear that a need exists for a cost effective liquid helium cooling system which allows examination of a sample crystal at a very low temperature without the problem of ice/solid air formation.